The Applicant has previously described a method of enabling users to access information from a computer system via a printed substrate e.g. paper. The substrate has a coding pattern printed thereon, which is read by an optical sensing device when the user interacts with the substrate using the sensing device. A computer receives interaction data from the sensing device and uses this data to determine what action is being requested by the user. For example, a user may make make handwritten input onto a form or make a selection gesture around a printed item. This input is interpreted by the computer system with reference to a page description corresponding to the printed substrate.
It would desirable to improve the coding pattern printed on the substrate so as to maximize usage of images captured by the sensing device, whilst still providing a robust error-correcting code. It would be further desirable to minimize visible coloration of the substrate by the coding pattern.